The invention relates to an apparatus for detecting motions of a lower jaw relative to the upper jaw of a vertebrate.
An apparatus and a method for determining all degrees of the freedom of motion and positions of the lower jaw relative to the upper jaw are described, for instance, in DE 10 2004 002 953 A1, which are based on the time delay measurement of ultrasonic pulses between transmitters and receivers of an ultrasonic system, with the aim to improve the measuring accuracy at significant virtual points, especially in the area of the jaw joint, with respect to the measured paths of motion and positions in space.
DE 35 00 305 A1 describes an apparatus for measuring the positions and motions of the lower jaw relative to the upper jaw. A first holder is fixed stationarily to the head of patient, and a second holder is provided at the lower jaw. The first holder comprises a plurality of receivers for ultrasonic pulses, and the second holder includes a plurality of ultrasonic transmitters which are arranged in a distributed manner. A transmitted ultrasonic pulse is successively received by the receivers of the first holder. The delay times of the transmitted ultrasonic pulses give information on the distances of an ultrasonic receiver from an ultrasonic transmitter, thereby allowing the exact detection and analysis of the motions of the lower jaw relative to the upper jaw. At the same time, all degrees of freedom of the system are uniquely detected.
These measuring systems are significant above all in the field of dentistry. For the production of dental prosthesis the teeth of the upper and lower jaws are used as models in articulators (mechanical motion simulators). This allows the reproduction of the motions of the patient's jaws and teeth, and prosthetic measures can be verified and optimized. These motions differ individually in every human being and depend significantly on the anatomy of the jaw joint. The exact detection of the individual motion sequences of the jaw joint, for instance, during chewing motions is important in order to minimize or avoid complex and cost-intensive follow-up treatments after the dental prosthesis or implant were implanted in the patient.
Document DE 10 2004 002 953 A1 of the applicant provides for an improved measuring system of this type, which allows in particular to obtain more exact measuring results.
So far, the detection and simulation of motion was based primarily on mechanistic model concepts as far as the motion sequences of the jaws relative to one another are concerned, especially since a mechanical articulator is by any account rigid and torsionally stiff in terms of construction and only contains rigid jaw models (usually made of plaster). Only the modern possibilities, which allow the scanning of jaws as a whole or of jaw sections, permit the consideration of biological and neurophysiological aspects in the simulation of motions.
It is known that the lower jaw brace bends when muscular strength is applied. If the mouth is widely opened—without much muscular strength—the lower jaw brace already bends inwardly in the area of the wisdom tooth, for instance by about 0.5 mm. These effects are even stronger in closing motions, namely always when occlusal forces are involved, e.g. during chewing or grinding/pressing of the teeth.
As one of the most crucial purposes of the chewing system is the comminution of food it is at any rate sensible and important to identify the effects of chewing forces on the bending of the lower jaw brace during the chewing, as this is the only way to construct chewing surfaces optimally with regard to the dynamic conditions in the chewing system in prosthetic-restorative measures. However, also the grinding and pressing of teeth—a frequent habit for stress reduction—may involve enormous forces which act uncontrollably for a longer period on teeth, jaws and joints, mainly at night, and result in abnormal functional and structural changes of the chewing system, especially in craniomandibular dysfunctions (CMD syndrome).
Therefore, it is the dentist's job to construct or arrange chewing surfaces or artificial teeth such that they are not only suited for the comminution of food, but also for the “reduction of stress”, i.e. that they are able to carry loads as optimally as possible or absorb and divert forces, thus protecting the jaw joints. Usually, this protective effect is obtained by specific occlusion concepts, e.g. the construction of a sequential tooth occlusion, where in lateral motions tooth surfaces are successively or sequentially loaded from the canine teeth to the side teeth. The dental technician tendentially makes the angle steeper than necessary in order to have “security reserves” which might be needed for bending the lower jaw, but which are unknown in the individual patient case. To provide a remedy in order to allow the construction of individual chewing surfaces is an important goal.
However, there is also a characteristic difference between the two jaw halves of the lower jaw during the chewing. Depending on the side where the chewing material is interposed between the rows of teeth a working side (=chewing material) and a balance side are formed. The working side is the side where the chewing material is located. During the chewing the lower jaw is initially opened slightly toward the opposite side and then pivots slightly to the working side until the maximum opening position is reached. In the closing motion the teeth grasp the chewing material initially with their cusp tips in a sideways pivoted cusp-to-cusp position and then comminute/squeeze the chewing material by the rows of teeth approaching one another when chewing force is applied. The rows of teeth slide along the cusp facets and cusp slopes into the scissor bite position of the initial toothing. The chewing material is successively comminuted in rhythmic cycles until it can be swallowed. The chewing muscles are equally active on both sides during the chewing motion, i.e. on the working side and the balance side, and bend the lower jaw brace during the comminution. The working side and the balance side virtually decouple themselves in the approaching motion of the chewing surfaces and would actually have to be recorded separately. For this is the only way how steepnesses of the cusp facets can be optimally constructed for the balancing motions and the working motions.
These dynamic aspects of the effect of biting forces can, so far, not be incorporated by the usual measuring systems. Merely the muscle activity as such is easy to detect by means of electromyographical measurements with surface electrodes which are placed on the skin above the muscle bellies. However, there are no measured values that allow the sensible detection and quantification of the individual deflections and deformations of the working side on the one hand and the balance side on the other hand. If mechanical motion simulators are used such additional information can hardly be considered. They demonstrate at best where the mechanical model is inadequate and to what extent and where dental restorations such as crowns, bridges and prostheses in the mouth cavity have to be corrected and adapted. As the future belongs to the so-called virtual articulators, however, which make the scanned rows of teeth visible on the computer screen and available in the computer, dynamic conditions and effects can be reasonably represented and implemented for the first time.
The dynamic behavior of the lower jaw brace, specifically the deformation resultant from force fit and occlusal force, plays a central role for the diagnosis and treatment planning relating to bony structures, e.g. the planing of implants, including the prostheses and crowns that have to be carried and supported by the implants. Implants are “osseointegrated” as artificial roots, in marked contrast to the parodontal, i.e. fibrous anchorage of natural teeth in the tooth bed. For this reason, implants have no or only a very small functional intrinsic mobility and, therefore, transfer occlusal forces directly to the jaw bone.
This is where information about the dynamic behavior of the lower jaw brace would be important. Implants would have to be inserted (in terms of biomechanics) at those positions and in that direction in which the local loads during chewing or grinding and pressing are minimized. To this end, initially the motions of the lower jaw brace would have to be represented as a whole.